1. Field of the Invention
This invention relates to the hydraulic control circuit of an automatic transmission and more particularly to such a control circuit for quickly pressurizing and quickly venting the cylinder of a clutch or servo through operation of a solenoid operated control valve.
2. Prior Art
It is known in the art that fluid capacity modulator valves and accumulators can operate together to smooth gear ratio upshift changes by controlling the rate at which the associated clutch is applied. When the upshift begins the capacity modulator valve produces a reduced or regulated pressure, which is applied to the operative clutch and to the spring of an accumulator. The accumulator is filled and pressure regulation continues until it is fully filled. Thereafter, regulation ceases and line pressure is directly applied to the clutch cylinder to complete the engagement. Such hydraulic systems provide gradual or rapid pressure build-up depending upon the conditions and the requirements indicated by movement of a control valve.
A desirable control feature of solenoid operated hydraulic control systems is optimized shift quality. It is undesirable to have a shift between gear ratios that extends for a very short time because it produces a noticable and objectionable rapid change in vehicle acceleration producing a harsh shift. On the other hand, if the period for engagement of the selected clutch or servo extends for too long, excessive heat and wear on the friction elements used to produce the desired coupling or braking of the gear elements results. Accordingly, the preferred engagement of a clutch or servo of an automatic transmission is long enough to avoid the objectionable harsh shifting that results when the engagement is too fast yet short enough to avoid excessive heat and wear that results if the engagement is unduly protracted.